Many controlled physical systems include rotating parts. Some examples include machine tools such as CNC lathes and routers as well as rotating data storage products for computers and consumer electronics. In particular, rotating storage devices include hard disk drives, CD players, CD-ROM drives, and DVD players. Accurate control over the position of the data read/write mechanism is critical to the operation of a rotating media storage product. For example, in a hard disk drive system data is stored in magnetic circular tracks on a media platter. Data is read and written by a magnetic head that floats over the circular tracks.
Unmeasured variables may affect the operation of a drive and can cause variations in performance from unit to unit, as well as within a unit during use of the drive. These variables may include temperature, spindle bearing asperity, disk flutter, induced non-repeatable run-out (NRRO), and variable repeatable run-out (RRO).
The rotating parts (disk platters and spindle-motor assemblies) in a rotating data storage device introduce periodic disturbances in the position of the read/write mechanism. In hard disk drives, for example, the track positions are recorded on each disk platter during a servo writing process. Ideally, the track positions are laid out in perfect concentric circles such that, while the disk is spinning, the head need not move since the data is located in a perfect circle relative to the head. However, small errors are made during the servo writing process that result in data tracks that are slightly non-circular. To a head position servo control system, this non-circularity appears as a periodic component in the error between the track position and the head position.
In the examples above and in many other cases the disturbance magnitude is a function of the angular position of the media platter under the read/write mechanism. Many rotating storage devices use sampled-data controllers with a fixed integer number of samples per full rotation of the storage media platter. The disturbances appear to such a sampled data system as signals that have a period that is an exact multiple of the sample time. The fundamental period of such signals is typically the number of samples per full rotation of the storage media platter. An important goal for a read/write mechanism position control system is to minimize the effects of these periodic disturbances.
Other systems suffer from the effects of periodic disturbances as well. Examples include electrical power delivery systems and communications systems that suffer from power line noise at harmonics of the AC power fundamental frequency.
All of these systems can benefit from a compensation scheme that minimizes the effects of periodic disturbances. FIG. 1 illustrates a prior art method for canceling periodic disturbances. This prior art method utilizes “plug-in”compensation where a repetitive compensator is used to augment an existing nominal compensator. FIG. 2 illustrates a prior art method for canceling periodic disturbances. This prior art method utilizes an adaptive feed forward cancellation scheme.
Both of the methods illustrated in FIGS. 1 and 2 have significant drawbacks. They only take as an input the error signal computed from either the plant output or the difference between the plant output and a desired setpoint. As a result, they do not differentiate between periodic disturbances and signals that are created by the feedback control system. The system in FIG. 1 is a feedback compensator and does not allow feed forward cancellation as does the system in FIG. 2. However, the prior art system of FIG. 2 only uses the output of their plant y(t) in its controller. Furthermore, the prior art system of FIG. 2 fails to account for the input/output behavior of the plant.
The RRO compensation methods are particularly difficult for CD-ROM drives. CD players and DVD drives because the error caused by RRO changes every time a disk is inserted into a drive. Furthermore, the error caused by RRO in CD-ROMs, DVDS, and CDs, changes over time based upon the changing rotational velocity of the disk, thus complicating RRO compensation methods.